1. Field of the Invention
The present invention relates to an optical modulator, and more particularly, to an optical modulator using a thin plate having an electrooptic effect.
2. Related Art Statement
In the related art, waveguide type optical modulators having optical waveguides or modulation electrodes formed on a substrate having an electrooptic effect have been widely used for optical communication fields or optical measurement fields.
Particularly, an amount of information transmission increases as multimedia technology is developed. Therefore, there is a need for a wide frequency band modulation technology in an optical modulator. In order to implement the wide frequency band modulation, various external modulation schemes with an LN (lithium niobate) modulator or the like are used. However, in order to implement the wide frequency band modulation with the LN modulator, there is a need for velocity matching between an optical wave and a microwave, that is, a modulation signal and decrease in driving voltage.
As one of means for solving the problem, a technique of satisfying the condition of velocity matching between the optical wave and the microwave and decrease in the driving voltage by using a thin substrate has been conventionally known.
In JPA_S64-18121 and JPA—2003-215519, an optical waveguide and an modulation electrode are disposed in a thin substrate (hereinafter, referred to as a first substrate) having a thickness of 30 μm or less, and after that, a substrate (hereinafter, referred to as a second substrate) having a dielectric constant lower than that of the first substrate is attached to the first substrate. By doing so, an effective refractive index with respect to a microwave is reduced, so that velocity matching between the microwave and an optical wave is obtained, and a mechanical strength of the substrate is maintained.
In JPA_S64-18121 and JPA—2003-215519, the first substrate is mainly made of LiNbO3 (hereinafter, referred to as LN), and the second substrate is made of a material having a dielectric constant lower than that of the LN, such as quartz, glass, and aluminum oxide. In the combination of these materials, due to a difference between linear expansion coefficients thereof, temperature drift occurs according to change in temperature or DC drift occurs. Particularly, in JPA—2003-215519, in order to remove such defect, the first and second substrates are attached to each other by using an adhesive having a linear expansion coefficient close to that of the first substrate.
However, in a case where an optical modulator using an LN substrate fabricated in the related art is compared with an optical modulator using a thin LN substrate, as the thickness of the substrate is reduced, the light radiating or leaking from an optical waveguide or the light from an incident optical fiber to a region excluding the optical waveguide (hereinafter, referred to as stray light) has a tendency to be confined to the substrate. Because the LN substrate in the related art has a large thickness (for example, 500 μm to 1000 μm), there is a sufficient region in which the optical waveguide having a depth of, for example, several micrometers is not influenced. In addition, a spatial distribution density of the stray light (hereinafter, referred to as a stray light density) is lowered. As a result, the influence of the stray light is not serious. However, in a case where the thickness of the substrate is about the depth of the optical waveguide, the stray light density in the substrate increases. Therefore, the stray light propagates inside the substrate to enter into the optical waveguide again or an output optical fiber connected to the optical modulator. As a result, the S/N ratio of the output light deteriorates.
Moreover, in a case where a thin plate is used for the optical modulator, since a sufficient mechanical strength cannot be obtained by using the thin plate, there is a need to attach the second substrate, that is, a reinforcement plate to the first substrate, that is, the thin plate by using an adhesive or the like as described above. In this case, if the refractive index of the adhesive is lower than that of the thin plate, the confinement of the stray light is greatly dominated.